The present invention relates to a printing quality examining apparatus (an apparatus for examining printing quality) used for decision as to whether printing quality is good or bad.
In a conventional printing quality examination method, as shown in FIG. 10, image data of printing paper are successively read in order of k=1, . . . , k=nxe2x88x921, k=n by means of a camera and the image data are averaged in time to obtain estimated value data. In FIG. 10, i, j and k are marks for defining coordinates for picture or pattern data on paper. The estimated value data is compared with a previously read reference data and when a difference therebetween exceeds a threshold value, pixels thereof are decided to be defective. A flow chart of this decision operation is shown in FIG. 11.
In the conventional printing quality examination method shown in FIGS. 10 and 11, however, an estimated value data which is an average value of image data on current printing paper and image data on past printing paper is compared with the reference data in order to improve detection accuracy. Accordingly, once any defect occurs, estimated value data of normal pixels on several printing papers used subsequently to the occurrence of the defect are influenced by defective data in the past, so that the normal pixels are decided as defective pixels in error and consequently even satisfactory paper is discharged as defective paper.
Further, FIG. 12 illustrates a printing quality examination apparatus used heretofore. This conventional printing quality examination apparatus is now described with reference to FIG. 12. In FIG. 12, numeral 105 denotes a printing paper put on an impression cylinder A so that the printing paper is curved arcuatedly, numeral 114 a camera disposed in a detection unit, numeral 115 a xenon lamp disposed far from the detection unit, numeral 116 an optical fiber extended from the xenon lamp 115, numeral 117 a light irradiating end formed at an end of the optical fiber 116, and numeral 118 an optical axis of the light source. The camera 114 is disposed so that an optical axis of the camera is substantially vertical to the printing paper 105 and the light irradiating end 117 is disposed so that an optical axis of the light irradiating end 117 is oblique to the printing paper 105.
The printing paper 105 is irradiated by light from the light irradiating end 117, while image data on the printing paper 105 is taken in by the camera 114 and defective paper is detected from the image data on the printing paper 105.
However, in the illumination apparatus of the conventional printing quality examination apparatus shown in FIG. 12, the printing paper 5 is illuminated by illumination light from the single light irradiating end 117 and accordingly the xenon lamp 115 having the high luminous intensity is required. Furthermore, since the xenon lamp 115 is disposed far from the detection unit and light from the xenon lamp is led to the detection unit through the optical fiber 116, a manufacturing cost thereof is increased.
Further, a condensing lens or the like is used to feed the illumination light from the xenon lamp 115 to the optical fiber 116 effectively. The condensing lens or the like must be maintained for the optical deterioration and a great deal of labor is required therefor.
In addition, at a held end of the printing paper 105 on the impression cylinder A, when image data is taken in by the detection unit, an amount of light incident on the detection unit is varied due to fluttering of the paper, so that amendment of the illumination light amount is influenced greatly. More concrete description is given as follows.
When normal printing paper is irradiated by illumination light vertically and an illuminometer 119 is moved in parallel to the printing paper to measure the luminous intensity on the paper while the illuminometer is maintained vertically and in an equal distance to the printing paper as shown in FIG. 13, the luminous intensity on the paper is expressed by a curve named a substantially normal distribution and the distribution of the luminous intensity is maximized in the vicinity of the optical axis 120 of the illumination.
In the illumination apparatus of the conventional printing quality examination apparatus, as shown in FIG. 14, illumination light is incident along an incident axis 121 oblique to the vertical axis and passing through an incident point of illumination light and the light is reflected along an emitting axis 122 having the same angle in the opposite direction. Accordingly, the maximum point of the luminous intensity on the paper is offset on the side of the reflected light with respect to the vertical axis as shown by a curve of FIG. 14.
FIG. 15 illustrates variation in the luminous intensity on the printing paper 105 due to a fluttering of paper occurring at the held end of the printing paper 105 on the impression cylinder A.
When fluttering of paper does not occur at the held end of the printing paper 105, the distribution of the luminous intensity shown by solid line is obtained and the luminous intensity on the paper on the optical axis of the camera 114 is I0. The case where the held end of the paper is moved up is now considered. An amount of variation or movement is regarded to be able to be neglected as compared with a distance from the light irradiating end 117 and the camera 114 to the printing paper 105 and the printing paper 105 is assumed to be angularly moved or rotated about an intersection point 0 of the printing paper and the vertical line drawn from the camera 114 to the printing paper. The upward movement of the held end of the paper corresponds to the angular movement of the printing paper 105 in the counterclockwise direction by an angle xcex1.
At this time, the normal line extending from the intersecting point 0 vertically to the printing paper 105 is rotated by xcex1 in the counterclockwise direction similarly. Further, the emitting axis of the reflected light is rotated by 2xcex1 and the maximum point of the luminous intensity in the distribution of the luminous intensity on the printing paper 105 is also moved leftward as shown by broken line in FIG. 15. Thus, an amount of light received by the camera 114 is varied from I0 to I1. This corresponds to a variation in the amount of light received by the camera 114 due to fluttering of paper.
Further, FIG. 16 schematically illustrates a circuit configuration of a conventional printing quality examination apparatus. In this case, an image of a printing paper having the luminous intensity on the surface thereof maintained constant by illumination light from an illuminating light source is taken in by a line camera of a detection unit 201. The luminous intensity on the printing paper is maintained to be constant and decision as to whether the printing paper is good or bad is made as follows.
As shown in FIG. 17, an output signal from the detection unit 201 is reduced in substantially inverse proportion to a machine speed. The output signal from the detection unit 201 is supplied to an amplifier 202 of FIG. 16 to be amplified and the amplified signal is supplied to an analog-to-digital (A/D) converter 203 to be converted to a digital signal. Thereafter, the digital signal is subjected to a correction process with respect to the machine speed and is held to be a fixed signal level as shown in FIG. 17. Then, the digital signal is supplied to a comparison operation circuit 204 of FIG. 16 to be compared with a previously taken-in reference image data. This operation result is supplied to a decision circuit 205 in which decision as to whether the printing paper is good or bad is made.
The decision result is supplied to the control and display unit 206 in which unsatisfactory paper is discharged and an alarm to an operator is displayed.
However, the printing quality examination apparatus shown in FIGS. 16 and 17 has the following problems.
The luminous intensity on the printing paper is maintained to be constant by illumination light from the illumination light source and since an amount of light received by the printing paper is reduced when the machine speed is increased, a gain is applied to the whole signal in order to maintain the received light amount constant. Accordingly, there are the following problems.
(1) Since a gain is applied to the whole signal, even noise is amplified.
(2) Variation of the received light amount depending on quality of paper cannot be understood.
(3) Since an amount of light is reduced due to a life of the illumination light source, it is necessary to maintain the illumination light source.
Further, in the conventional printing quality examination apparatus, as shown in FIG. 18, an image data of a blank portion 311 of the printing paper is taken in by the camera of the detection unit and brightness of this portion is compared with a previously set reference value of the luminous intensity. An amount of light of the illumination light source is corrected on the basis of the result thereof to thereby obtain the luminous intensity required in the detection unit.
However, in the conventional printing quality examination apparatus shown in FIG. 18, it is presupposed that the blank portion 311 for correction of the light amount is present in the printing paper. However, some printing papers have no blank portion and in this case, it is impossible to correct the light amount.
On the other hand, when the blank portion 311 necessary for the correction of the light amount exists in the vicinity of a picture portion 312 and the picture portion is moved or shifted due to mechanical vibration or the like, an amount of light in the printing portion other than the blank portion 311 is sometimes detected. When the light amount is corrected in such a state, there is a problem that a corrected light amount in the detection portion becomes instable.
The present invention has been made in view of the above various problems and objects thereof are as follows:
It is a first object of the present invention to provide a printing quality examining method capable of eliminating a defect that some satisfactory papers subsequent to defective paper are decided as defective papers.
Further, it is a second object of the present invention to provide an illumination apparatus of a printing quality examining apparatus capable of (1) improving defect detection accuracy of printing paper extremely, (2) reducing a manufacturing cost and reducing labors required to maintain optical parts.
In addition, it is a third object of the present invention to provide a printing quality examining apparatus capable of (1) improving defect detection accuracy of printing paper extremely, (2) detecting defect of printing paper with high stability, and (3) reducing labors required to maintain an illumination light source.
Furthermore, it is a fourth object of the present invention to provide a printing quality examining apparatus capable of (1) performing correction of an amount of light to even printing paper having no blank portion, (2) stabilizing a corrected light amount in a detection portion, and (3) reducing a cost of printing paper when the paper is expensive.
In order to achieve the first object, according to the present invention, in the printing quality examining method in which image data of a printing paper successively taken in by a camera of a detection unit is compared with a previously taken-in reference data to detect a printing defect and decide whether the printing quality is good or bad, the image data of the printing paper taken in by the camera of the detection unit is averaged in time to calculate estimated data and the estimated data is compared with previously taken-in reference data. When the estimated data is normal, pixels are decided to be normal and when the estimated data is unusual, the image data from the detection unit is compared with the reference data at the next step. When the image data from the detection unit is normal, the pixels are decided to be normal and when the image data is unusual, the pixels are decide to be defective and the paper is discharged.
In a preferred aspect of the present invention, the estimated data and the image data having characteristics complementary to each other are compared with the reference data and decision as to whether the printing quality is good or bad is made on the basis of the resultant logic product.
According to the present invention with the above structure, only the unusual printing paper is decided to be unusual and there is no case where some satisfactory papers subsequent to defective paper are decided to be defective notwithstanding satisfactory paper.
Further, in order to achieve the second object, according to the present invention, in the printing quality examination apparatus in which a detection unit including illumination light sources and a camera is disposed in opposite to an arcuatedly curved printing paper and a defective paper is detected from image data of the printing paper taken in by the detection unit, there is provided two illumination light sources and the two illumination light sources are disposed so that axes of illumination light from the two illumination light sources are symmetrically oblique to an optical axis of the camera and have equal angles with respect to the optical axis of the camera.
With the above structure, in the printing quality examining apparatus of the present invention, when the printing paper is rotated counterclockwise by xcex1, an emitting axis of one illumination light source is also rotated in the same direction by 2xcex1. Accordingly, the maximum point of the distribution of luminous intensity on the paper by the one illumination light source is moved leftward and an amount of light on the optical axis of the camera is increased in the same manner as the prior art. However, the maximum point of the distribution of luminous intensity by the other illumination light source is also moved leftward to thereby reduce the amount of light on the optical axis of the camera. In this manner, since the light amounts of both the illumination light sources are increased and reduced relatively, the illumination apparatus always emits a fixed amount of light with respect to the camera even if the printing paper is varied due to fluttering of the held end thereof.
Furthermore, in order to achieve the third object, according to the present invention, in the printing quality examining apparatus including a line camera of a detection unit for taking in image data of printing paper, an illumination light source for illuminating the printing paper to ensure an amount of light required by the line camera, a camera controller for controlling the line camera, and an examination controller for comparing the image data obtained by the line camera with reference image data to decide whether the quality of the printing paper is good or bad, the printing quality examining apparatus comprises a light amount correction unit for varying an amount of light of the illumination light source in accordance with printing speed, paper quality, the light source and the like and holding a received light amount of the line camera to be constant, so that a signal level of the image data is corrected.
In a preferred aspect of the present invention, the light amount correcting unit comprises a light amount difference detection unit for comparing image data of a blank portion of the printing paper with a previously set target luminous intensity and a light amount difference correction circuit for obtaining a correction value for a difference of the luminous intensity from the light amount difference detection unit.
According to the present invention with the above structure, the blank portion of the printing paper is read by the line camera of the detection unit and an image data of the blank portion is supplied to the light amount correction unit in which the image data of the blank portion is compared with the previously set target luminous intensity to obtain the difference of light amount. A correction voltage for correcting a voltage value of a power supply for the light source is obtained while using the difference of light amount as a parameter. The power supply for the light source is controlled by the correction voltage to change the luminous intensity of the illumination light source, so that a signal having a fixed level is always obtained from the detection unit without relation to the machine speed. That is, when the machine speed is increased and a received light amount of the detection unit is reduced, the illumination light source is controlled so that the illumination light amount can be increased correspondingly to maintain the received light amount from the blank portion of the detection unit to be always constant.
In addition, in order to achieve the fourth object, according to the present invention, in the printing quality examining apparatus in which printing paper is illuminated by an illumination light source and image data of the printing paper is taken in by a camera of a detection unit to decide whether the printing quality is good or bad in accordance with the image data, the printing quality examining apparatus comprises a maximum light amount pixel detection circuit for selecting a plurality of maximum light amount pixels from image data taken in by the camera and determining and holding pixel positions thereof, a light amount averaging circuit for calculating an average value of the maximum light amount pixels, and a difference circuit for comparing the average value with a previously set reference value of light amount to calculate a difference thereof. Further, in the present invention, when the printing paper is illuminated by the illumination light source and the image data of the printing paper is taken in by the camera of the detection unit to decide whether the printing quality is good or bad on the basis of the image data, the plurality of maximum light amount pixels are selected from the taken-in image data by the maximum light amount pixel detection circuit and the pixel positions thereof are determined and held. The average value of the maximum light amount pixels is calculated by the light amount averaging circuit and the average value is compared with the previously set reference value of light amount by the difference circuit to calculate a difference thereof. A voltage variation is calculated while using the difference as a parameter and the light amount of the illumination light source is corrected in accordance with the voltage variation to obtain the optimum illumination condition by the detection unit.